Aromatic vinyl-conjugated diene-aromatic vinyl block copolymers such as styrene/isoprene/styrene block copolymers (SIS) and styrene/butadiene/styrene block copolymers (SBS) are thermoplastic elastomers having characteristic properties in various aspects, and are therefore used in various applications. Of thermoplastic elastomers, aromatic vinyl-conjugated diene-aromatic vinyl block copolymers, in particular, are highly elastic and flexible, and are used as a material for flexible films used for sanitary goods such as paper diapers and sanitary protections for one typical application thereof.
Flexible films are used as various parts of sanitary goods such as paper diapers and sanitary protections, as they are required to have followability and fittability to the movement of wearers. For example, in training pants (pull-ups), a type of paper diapers, a flexible film is disposed in the openings around both legs, in the opening around the waist and in both the lower backs. The sanitary goods must not be out of alignment even when the wearers move violently or when they are worn for a long period of time, and therefore the flexible film to be used in such applications is required to have both a high elastic modulus and a small elongation set. However, it is hard to say that conventional aromatic vinyl-conjugated diene-aromatic vinyl block copolymers could satisfy both these characteristics. Accordingly, various investigations have been made for improving the elastic modulus and the elongation set of aromatic vinyl-conjugated diene-aromatic vinyl block copolymers.
For example, Patent Reference 1 discloses that an elastomer polymer blend composition comprising from 65 to 92 parts by weight of a specific elastomer monovinylidene aromatic/conjugated diene block copolymer having a monovinylidene aromatic content of less than 50% by weight, and from 8 to 35 parts by weight of a specific thermoplastic monovinylidene aromatic/conjugated diene block copolymer having a monovinylidene aromatic content of at least 50% by weight gives elastomer products showing excellent elasticity and stress relaxation characteristics.
Patent Reference 2 discloses a flexible film, which is produced through extrusion of a composition prepared by incorporating a specific polyisoprene or the like with a specific aromatic vinyl-conjugated diene block copolymer, is anisotropic, excellently flexible and favorable for use as members for sanitary goods such as paper diapers and sanitary protections.
However, even the techniques described in these references are still unsatisfactory in point of satisfying a high-level balance between a high elastic modulus and a small elongation set, and are required of further improvements.